Pulverized coal boiler generator set, as the core technology of thermal power generation, experiences one hundred years of development history. From the subcritical to the supercritical, then to the ultra-supercritical, China's coal-fired power technology gets a rapid development in recent years. The rapid development of ultra-supercritical coal-fired power technology and the improvement of unit efficiency are the most cost-effective way to realize energy saving and emission reduction and to reduce carbon dioxide emission.
At present, the generating efficiency of a subcritical single-reheat thermal power generating unit is about 37%, and the generating efficiency of a supercritical single-reheat thermal power generating unit is about 41%, and the generating efficiency of an ultra-supercritical single-reheat thermal power generating unit with the temperature of main steam and reheated stream of 600° C. is about 44%; if the steam parameter is further improved, the unit generating efficiency is expected to be further increased. For example, when the temperature of main steam and reheated stream reaches 700° C. or above, the generating efficiency of a single-reheat thermal power generating unit is expected to reach above 48.5%, and the generating efficiency of a double-reheat thermal power generating unit is expected to reach above 51%. Therefore, an advanced ultra-supercritical thermal power generating unit technology with steam temperature reaching or exceeding 700° C. is actively carried out in China, European Union, US and Japan.
The development of an advanced ultra-supercritical thermal power generating unit with ultrahigh steam parameters (the temperature of main steam and reheated steam reaches 700° C. or above) confronts with many important technical problems; in which, the major technical difficulty includes two aspects; one aspect is to develop a super alloy material meeting the application requirement of the advanced ultra-supercritical thermal power generating unit of ultrahigh steam temperature reached 700° C.; the other aspect is to realize the design optimization of the unit system and to reduce the manufacturing cost.
The research shows that the super alloy material most likely to be used for the high-temperature part of the ultra-supercritical thermal power generating unit mainly is a nickel base alloy. However, the nickel base alloy material is very expensive, more than 15 times of the price of a present common iron base heat resistant alloy steel of level 600° C. According to the system deployment mode of a present common thermal power generating unit, if the nickel base alloy material is adopted, taking two 1000 MW ultra-supercritical units for example, just the cost of the four high-temperature pipelines between the main steam/reheated steam and a steam turbine would be increased to about 2.5 billion RMB from the present 300 million RMB. In addition, the manufacturing cost is increased when the high-temperature parts of the boiler and the steam turbine adopt a heat resistant alloy, finally the overall cost of the advanced ultra-supercritical unit of level 700° C. would be greatly higher than that of the thermal power generating unit of level 600° C., which limits the application and promotion of the advanced ultra-supercritical thermal power generating unit.
In addition, the common thermal power generating unit with the temperature of main steam and reheated steam of 600° C. or below can adopt a method of single-reheat or double-reheat steam. Although the double-reheat method can improve the unit efficiency to a great extent, the complexity of the unit system adopting the double-reheat technology is higher than that of the unit system adopting the single-reheat technology and the investment thereof is greatly increased, which limits the application of the double-reheat system. At present, most of the large-scale thermal power generating units adopts the single-reheat system, and few large-scale thermal power generating units adopt the double-reheat system. If the complexity and manufacturing cost of the double-reheat system can be reduced by optimizing the design of the unit system, the realistic feasibility of the large-scale thermal power generating unit adopting the double-reheat system would be greatly improved.
Therefore, the point on how to optimize the design of the unit system and reduce the consumption of a high-temperature material (for example, four pipelines) plays a great role in implementing the application and promotion of the ultra-supercritical unit of ultrahigh steam temperature, promoting the application of the double-reheat system to a large-scale thermal power generating unit and improving the generating efficiency of the unit.
A Chinese patent “A novel steam turbine generating unit” with patent number of 200720069418.3 discloses a method for reducing the length and cost of a high temperature and high pressure steam pipeline of a double-reheat unit by distributing a high shafting and a low shafting at different height; however, since the high shaft formed by a high pressure cylinder and a generating unit needs to be arranged at a height of about 80 meters, serious problems such as shaking might be caused, and it is needed to solve the technical problems of support and foundation, thus this arrangement method has not been applied.
At present, the pulverized coal boilers generally adopt an arrangement mode of π-type boiler or tower type boiler, and a few adopt a T-type boiler, in which, the π-type boiler is the most common boiler arrangement mode adopted by the large/middle-scale thermal power generating unit. As shown in FIG. 1, the boiler consists of a hearth and a tail flue, and part of heating surfaces is arranged in a horizontal flue and a shaft of the tail flue. When the boiler is arranged in a form of π, the height of the hearth is shorter than that of the tower type boiler; therefore, the π-type boiler is good for the areas with strong earthquake and strong wind, with low manufacturing cost. However, since the eddy and disturbance of the flue gas is severe, the flow uniformity of the flue gas is poor, and it is easy to cause uneven heating of the heating surfaces, thus great temperature deviation is caused; and the boiler is heavily abraded when inferior fuel is combusted.
In a tower type boiler, all heating surfaces are arranged above the hearth, and the tail downward vertical flue is not provided with a heating surface, as shown in FIG. 2. Compared with the π-type boiler, the area occupied by the tower type boiler is smaller, which is suitable for the project with factory lacking land. Since the flue gas of the tower type boiler flows upwards, the dust in the flue gas flows slower and slower or sinks under gravity, thus the abrasion of the heating surfaces is greatly reduced. Besides, since the flue gas has good flow uniformity, the temperature deviation of the heating surfaces and working medium is smaller. Further, the tower type boiler has a simple structure, and the inflation center and the seal design of the boiler are easy to process, and the arrangement is compact; therefore, for the ultra-supercritical unit, the tower type boiler has certain advantages.
As for the T-type boiler, the tail flue is divided into two convection shaft flues of the same size, wherein the two convection shaft flues are arranged at two sides of the hearth symmetrically, as shown in FIG. 3, so that the problem of difficult arrangement of the tail heating surface occurred in the π-type boiler can be avoided, the height of the outlet smokestack of the hearth can be reduced to reduce the thermal deviation of the flue gas along the height; besides, the flow rate of the flue gas in the shaft can be reduced to reduce abrasion. However, the area occupied by the T-type boiler is greater than that occupied by the π-type boiler, the gas-water pipeline connection system is complex and the metal consumption is big, thus the T-type boiler is less applied.
No matter what arrangement mode the boiler adopts, due to the need of heat transfer, the high-temperature heating surfaces need to be arranged at an area with high flue gas temperature, while the elevation of the position on which the area with high flue gas temperature is located is high (above 50 to 80 meters), thus the high-temperature steam connection pipeline between the high-temperature heating surface outlet and the steam turbine is very long (for example, for the tower type boiler, the length of a single high-temperature steam pipeline reaches 160 to 190 meters), and the cost is high, and the application of the double-reheat technology is limited. When the steam temperature reaches 700° C., since the material cost per unit weight of the high-temperature steam connection pipeline is greatly increased more than 10 times), the point on how to reduce the length of the high-temperature steam connection pipeline and reduce the usage amount of the high-temperature steam connection pipeline so as to reduce the manufacturing cost of the high-temperature boiler becomes a key technical problem to be solved.
Besides, it takes a relatively long time to burn out the pulverized coal in the hearth, thus a relatively high hearth height is needed; however, the increase of the hearth height means the great increase of the manufacturing cost. Thus, the point on how to prolong the burning time and improve the burnout degree of the pulverized coal particles in the case of not increasing the hearth height also becomes a long-term concerned technical problem in the technical field of boilers.